Image recognition system are generally known. By way of example, U.S. Pat. No. 5,267,332 describes a method and apparatus in which images are characterized and compared on the basis of internal structure, which is independent of image size and image orientation. A library of reference images is first generated and stored, then each input image, or test image, is compared to the images stored in the library until a match is found. The image is represented in memory as nodes, lines, and curves. A plurality of descriptors, called reference keys and reference series, are generated for both the reference images and the test image. The reference library is screened for likely matches by comparing the descriptors for the test image to the descriptors in the reference images in the library. Inclusionary and exclusionary tests are performed. After screening, each candidate reference image is searched by comparing the pathway through the reference image and the pathway through the test image, and by the degree of correlation between the reference and test images. In addition, the link ratio, a measure of the portion of the test image actually matched to the reference image is computed.